The Liberal Democrats have moved to defuse a potentially damaging internal row after Danny Alexander said the party would fight the next general election committed to £15bn in spending cuts for 2016 and 2017 that had been agreed with the Tories.

In remarks that caused concern to senior Lib Dem colleagues, the chief secretary to the Treasury said the parties would set out "well before the next general election" how they would deliver the extra savings in the next parliament.

Pressure to set out post-election spending plans has been forced on the parties by the deteriorating fiscal position set out by George Osborne in his autumn statement. The chancellor said lower growth meant a further £15bn of cuts would be needed in 2016 and 2017 to eliminate the structural deficit within five years.

"As a government we originally set out plans that would meet our targets a year early in 2014/15," Alexander said on BBC 2's Newsnight. "But because of the way that economic circumstances have deteriorated we need to make this commitment for future years, so yes Liberal Democrats and Conservatives will work together in government to set out plans for those following two years and, of course, we will both be committed to delivering them.

"We have just decided what the path of spending is in the following two years [after the election] and in due course will set out what that means in detail."

Simon Hughes, the Lib Dem deputy leader, questioned the idea that the party could be committed to specific spending cuts after 2015. "All governments need to make spending plans for the longer term, but any spending plans from the government after 2015 can only be provisional and subject to the result of the general election," he said.

"The Liberal Democrats will fight the next election on an independent manifesto which will be developed through our internal democratic structures, and without any collaboration or agreement with other political parties."

It is understood that Alexander's remarks also caused concern among Lib Dem cabinet members.

Alexander insisted the two parties would have separate election manifestos but his remarks imply they will not simply have the same fiscal objectives, but an agreed programme to meet those objectives.

The party said: "At the 2015 election the Liberal Democrats will fight as an independent party that delivers economic credibility and greater fairness.

"The coalition government's autumn statement confirmed the spending totals for the first two years of the next parliament. We have not as a coalition government taken the decisions on the detailed breakdown of spending in these years, or even when those decisions will be taken."

A spokesman suggested the two parties might settle a spending review before the election setting out how the cuts would be made, but neither party would be committed to these spending plans in the election.

The episode shows how the delay in eradicating the deficit makes it more difficult for the two parties to disengage before the election.

Asked what was the point of voters backing the Lib Dems, Alexander said: "There are a lot of points to vote Liberal Democrat rather than Conservative or Labour, but those are things that we will set out at the general election in our respective manifestos."

Lord Oakeshott, a Lib Dem peer and ally of the business secretary, Vince Cable, questioned Alexander's statement, saying: "The coalition agreement is for five years not seven. It is very dangerous for the Liberal Democrats to go into the next election tied like a tin can to the tails of the Tories, rather than as an independent party capable of forming an alliance with whoever we and the electorate choose. In the economic circumstances of the 20s and 30s, the Tories proved themselves to be past masters at swallowing up other parties. It would be very foolish if we were to make the same mistake again."

Some Lib Dems argue that the two parties would lose credibility with the markets if details of spending cuts were withheld, adding that the party at the last election attacked Labour for failing to specify what would be cut. The current spending review ends months before the 2015 election.